Rex says
Everyday utility math — the kind you'd otherwise pull up four browser tabs for. I keep it to one clean answer.
Try a scenario
Click to load — tweak from there.Inputs
Result
Commit to
29.9
Overrun-adjusted days
26.0
Buffer added
9.9
Psst — share this and help Rex grow
One click, a permanent link with your numbers baked in.
How to use this
- 1Enter raw estimate (days).
- 2Enter historical overrun (%).
- 3Enter risk buffer (%).
- 4Read your commit to on the right — it updates as you type.
- 5Hit Share to keep the scenario or send it to someone.
About this calculator
Estimates slip. This applies an optimism factor and a risk buffer to your raw estimate so the date you promise is a date you can hit.
Worked example
Using the values the calculator loads with:
Inputs
- Raw estimate: 20 days
- Historical overrun: 30 %
- Risk buffer: 15 %
Results
- Commit to: 29.9
- Overrun-adjusted days: 26
- Buffer added: 9.9
What each field means
Inputs
- Raw estimate (days)
- The raw estimate used in the calculation, measured in days. Starts at 20 days so you have a working example on load.
- Historical overrun (%)
- The historical overrun used in the calculation, measured in %. Starts at 30 % so you have a working example on load. Accepted range: 0–200 %.
- Risk buffer (%)
- The risk buffer used in the calculation, measured in %. Starts at 15 % so you have a working example on load. Accepted range: 0–100 %.
Results
- Commit to
- Returned as a decimal number and shown as the headline result. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Overrun-adjusted days
- Returned as a decimal number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Buffer added
- Returned as a decimal number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
FAQ
How does the project buffer estimator work?
Estimates slip. This applies an optimism factor and a risk buffer to your raw estimate so the date you promise is a date you can hit. The underlying maths is: Committed = estimate × (1 + overrun) × (1 + risk).
What do I need to enter?
3 values: raw estimate, historical overrun, and risk buffer. Each field starts with a sensible default, so you can change one number at a time and watch the result move.
What does the commit to result mean?
It is returned as a decimal number and updates live as you edit the inputs, so you can compare two or three versions of a scenario in a few seconds.
Is this calculator free, and do I need an account?
Yes, it's free, and no account is required. Nothing you type is stored on our servers — the maths runs entirely in your browser.
How accurate is the result?
It applies the standard formula exactly, so the arithmetic is precise. Estimates assume standard, average conditions — local rules, pricing, and materials vary.
Who is this tool for?
It's built for anyone doing everyday planning maths — a quick estimate that would otherwise need four browser tabs and a calculator app.
Accuracy and limitations
- Estimates assume standard, average conditions — local rules, pricing, and materials vary.
- Results are rounded for readability; add a buffer before ordering, booking, or committing.
- Double-check anything with a real cost attached against a local quote.
Related tools
Cite this calculator
Writing about this topic? Grab a citation — every link helps keep these tools free.
RevenueLab. (2026). Project Buffer Estimator. Retrieved from https://www.revenuelab.fyi/toolbox/project-buffer-estimator
<p>Source: <a href="https://www.revenuelab.fyi/toolbox/project-buffer-estimator" target="_blank" rel="noopener">Project Buffer Estimator — RevenueLab</a> (2026).</p>
Source: [Project Buffer Estimator — RevenueLab](https://www.revenuelab.fyi/toolbox/project-buffer-estimator) (2026).